Magnetic field characterization of stresses and properties in materials

ABSTRACT

Described are methods for monitoring of stresses and other material properties. These methods use measurements of effective electrical properties, such as magnetic permeability and electrical conductivity, to infer the state of the test material, such as the stress, temperature, or overload condition. The sensors, which can be single element sensors or sensor arrays, can be used to periodically inspect selected locations, mounted to the test material, or scanned over the test material to generate two-dimensional images of the material properties. Magnetic field or eddy current based inductive and giant magnetoresistive sensors may be used on magnetizable and/or conducting materials, while capacitive sensors can be used for dielectric materials. Methods are also described for the use of state-sensitive layers to determine the state of materials of interest. These methods allow the weight of articles, such as aircraft, to be determined.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/292,146, filed Nov. 30, 2005, which is a divisional of U.S.application Ser. No. 10/441,976, filed May 20, 2003, which claims thebenefit of U.S. Provisional Application No. 60/382,447, filed May 21,2002,U.S. Provisional Application No. 60/384,006, filed May 28, 2002,and U.S. Provisional Application No. 60/388,103, filed Jun. 11, 2002.

The entire teachings of the above applications are incorporated hereinby reference.

GOVERNMENT SUPPORT

The invention was supported, in whole or in part, by a grantF09650-01-M-0956 from the U.S. Air Force. The Government has certainrights in the invention.

BACKGROUND OF THE INVENTION

The technical field of this application is that of nondestructivematerials characterization, particularly quantitative, model-basedcharacterization of surface, near-surface, and bulk material conditionfor flat and curved parts or components using magnetic field based oreddy-current sensors. Characterization of bulk material conditionincludes (1) measurement of changes in material state, i.e.,degradation/damage caused by fatigue damage, creep damage, thermalexposure, or plastic deformation; (2) assessment of residual stressesand applied loads; and (3) assessment of processing-related conditions,for example from aggressive grinding, shot peening, roll burnishing,thermal-spray coating, welding or heat treatment. It also includesmeasurements characterizing material, such as alloy type, and materialstates, such as porosity and temperature. Characterization of surfaceand near-surface conditions includes measurements of surface roughness,displacement or changes in relative position, coating thickness,temperature and coating condition. Each of these includes detection ofelectromagnetic property changes associated with either microstructuraland/or compositional changes, or electronic structure (e.g., Fermisurface) or magnetic structure (e.g., domain orientation) changes, orwith single or multiple cracks, cracks or stress variations inmagnitude, orientation or distribution.

A specific application of these techniques is the inspection ofhigh-strength steel components with the goal of measuring applied andresidual stresses and detecting early stage fatigue damage or hydrogenembrittlement. Highly stressed aircraft components, such as landing gearcomponents, require the use of steels such as 4340M and 300M heattreated to very high strength levels. The integrity of these componentsis critical to the safe operation of aircraft and for maintainingreadiness of military aircraft. However, unintentional loading of thesecomponents, such as a hard landing, can impart residual stresses thatcompromise the integrity of the component. Similarly, the mechanicalproperties of these ultra-high strength steels can be seriously degradedas a result of the ingress of hydrogen. Hydrogen ingress can occurduring pickling or plating operations and also during cleaning withcitric acid based maintenance solutions. The resulting hydrogenembrittlement is unpredictable and can cause catastrophic failure of thecomponent. Hydrogen embrittlement has been established as the directcause of numerous landing gear failures. This similarly applies torelated degradation mechanisms such as temper embrittlement, creep andother degradation processes that reduce a materials functional behavior.

The detrimental effects of hydrogen on material properties and componentintegrity have been observed in a wide range of metals, as described forexample in Interrante and in Hydrogen in Metals. Management ofhigh-strength steel components embrittled by hydrogen is made moredifficult by the fact that failures are typically delayed, occurringsome time after ingress of atomic hydrogen. The delay between exposureto hydrogen and failure of a high strength steel component depends on anumber of factors. Among these are the levels of hydrogen concentration,tensile stress, temperature, stress gradients, and certain impurities inthe steel, as well as the type, concentration, and size of certaincrystal lattice defects and inclusions. Moreover, susceptibility tohydrogen embrittlement can vary significantly between different heats ofsteels and between different pours from a given heat, as described byLawrence. Hydrogen concentration on the order of a few parts per millionis sufficient to cause hydrogen embrittlement and delayed fracture. Onceatomic hydrogen enters the steel, excess hydrogen atoms diffuse toinclusions, preexisting defects, and zones of high dislocation density.Some hydrogen atoms, as a result of stress-assisted diffusion, cancluster and form “platelets” leading to initiation of microcracks. Whensuch platelets form in front of a crack tip, they facilitate crackextension. Critical regions where hydrogen cracks are more likely toinitiate are notches or other stress raisers where local hydrogenconcentration is higher due to enhanced diffusion into the triaxiallystressed region in front of a stress raiser. Cracks at these criticallocations often initiate close to but beneath the surface, making themmore difficult to detect.

A recent review of existing magnetic/electromagnetic, diffraction,ultrasonic and other methods for assessment of residual stresses insteel components by Bray highlighted strengths and weaknesses of theavailable methods. This review also indicated that practical andcost-effective methods for assessment of residual stresses as well asfor monitoring of applied stresses over wide areas in steel componentsare not yet available. Typically, discrete strain gages are mounteddirectly onto the material under test (MUT). However this requiresintimate fixed contact between the strain gage and the MUT andindividual connections to each of the strain gages, both of which limitthe potential usefulness for monitoring stress over large areas.Furthermore, strain gages are limited in durability and do not alwaysprovide sufficient warning of gage failure or malfunction. Possiblecorrelations between magnetic properties and stresses in ferromagneticmaterials have been studied for over 100 years, as reviewed by Bozorth.Magnetostriction effect data suggests that, depending on the magnitudeand sign of the magnetostriction coefficient, correlation between stressand magnetic permeability within certain ranges of the magnetic fieldshould be present. However, attempts to use conventional inductive,i.e., eddy-current sensors for assessment of residual stresses as wellas for a number of other applications have shown serious limitations,particularly for complex geometry components.

Conventional eddy-current sensing involves the excitation of aconducting winding, the primary, with an electric current source ofprescribed frequency. This produces a time-varying magnetic field at thesame frequency, which in turn is detected with a sensing winding, thesecondary. The spatial distribution of the magnetic field and the fieldmeasured by the secondary is influenced by the proximity and physicalproperties (electrical conductivity and magnetic permeability) of nearbymaterials. When the sensor is intentionally placed in close proximity toa test material, the physical properties of the material can be deducedfrom measurements of the impedance between the primary and secondarywindings. Traditionally, scanning of eddy-current sensors across thematerial surface is then used to detect flaws, such as cracks.Conventional eddy-current sensors widely used in nondestructive testingapplications are effective at examining near surface properties ofmaterials, but have a limited capability to examine material propertyvariations deep within a material. In contrast, ultrasonic techniquesthat are also widely used are effective at measuring property variationsdeep within a material, but have limited sensitivity near the surfaceand behind some geometric features such as air gaps.

SUMMARY OF THE INVENTION

Aspects of the embodiment of the invention described herein involvenovel sensors and sensor arrays for the measurement of the near surfaceproperties of conducting and/or magnetic materials. These sensors andarrays use novel geometries for the primary winding and sensing elementsthat promote accurate modeling of the response and provide enhancedobservability of property changes of the test material.

Methods are described herein for the monitoring of material propertiesas they are changed during processing. This can involve disposing aneddy current sensor or sensor array in proximity to the test materialand converting the response of each sensor or sense element into aneffective material property. In one embodiment, the sense elements aresensing coils the respond to absolute changes in the magnetic fieldresponse. Preferably, these coils are rectangular. In anotherembodiment, the sense elements incorporate GMR sensors. In a preferredembodiment, the leads to the sense elements also have a proximate set ofleads that permit cancellation of the stray magnetic flux to the leadsand permits the use of small sense elements. In one embodiment of theinvention, the process being monitored is the heat treatment of amaterial. In one embodiment, the effective property is the electricalconductivity. In another, it is the lift-off. In one embodiment, themeasurements are performed at multiple excitation frequencies. Inanother embodiment, the sensor is not in contact with the surface of thetest material, which helps to minimize any effects the monitoring systemmay have on the environment around the material being processed.

Another aspect of the invention includes methods for monitoring stressesin materials. Preferably, this is performed with eddy current sensorsdisposed in proximity to the surface of the test material or embeddedwithin layers of the test material. In one embodiment, measurements areperformed with multiple orientations of the sensor relative the stressdistribution. In another embodiment, the measurement orientations areperpendicular. Preferably, the orientations correspond to the maximumand minimum principal stresses, or to maximum and minimum propertyvalues that are to be determined. In yet another embodiment, the sensoror sensor array is mounted on a flexible substrate. The sensor can havea foam backing, which permits sensor conformability to the test materialsurface, or a rigid backing that approximates the shape of the materialsurface and permits contact or non-contact measurements. In oneembodiment, the sensor has a plurality of sense elements which may bealigned with one another. In another embodiment, the sensor is scannedover the surface, preferably in multiple orientations, so that theentire stress distribution and orientation of stresses over the materialcan be resolved. In a preferred embodiment, the sensor arrays arescanned in two mutually perpendicular orientations, preferably with theorientations corresponding to the directions of maximum and minimumprincipal stresses.

Alternatively, the sensors or sensor arrays can be mounted in one ormore locations to monitor the stresses. When at least two sensors aremounted in different locations, the sensors can have differentorientations. Preferably, the orientations are mutually perpendicular,which also permits the monitoring of maximum and minimum principalstresses. In one embodiment, stress measurements are performed atmultiple excitation frequencies. In another embodiment, the electricalproperty of interest is the magnetic permeability.

Other aspects of the invention include methods for the inspection ofmagnetic materials. In one embodiment, an eddy current sensor is placedproximate to a test material to measure the magnetic permeability. Inone embodiment, the sensor response is measured at multiple locations todetermine the magnetic permeability distribution of the materialsurface. In another embodiment, the permeability is measured withdifferent sensor orientations, in another it is measured with sensorsplaced at different locations. In one embodiment, the sensor has aplurality of sense elements that may be aligned with one another. Inanother embodiment, the sensor is scanned over the surface, preferablyin multiple orientations, so that the material property distribution andorientation over the material can be resolved. In another embodiment,the sensor array is mounted to the surface. Preferably, at least onemore sensor array is mounted with a different orientation.

Yet another method is described for the monitoring of the weight of anarticle, by measuring the magnetic permeability of a portion of the testmaterial that transfers the mechanical load from the article andcorrelating this permeability with the article weight. Preferably, thearticle is an aircraft. In one embodiment, measurements are performed atseveral select locations on the material. In one embodiment, the sensorsare mounted to the material In another, the sensors are scanned over thesurface of the material. Preferably, the sensors are scanned so that thepermeability is measured in a direction that is parallel to thedirection of maximum principal stress.

Also described are methods for monitoring of the state of a material ora test article. In one embodiment, a sensor is placed in proximity to atest material to measure an electrical property of the state of thearticle. In one embodiment, the electrical property is the magneticpermeability and in another the electrical property is the electricalconductivity. In one embodiment, the state is stress and in another itis temperature. In another embodiment the state is an overloadcondition. Preferably, this overload condition results from excessivetemperature expose as a thermal overload or from over stressing as amechanical overload. Alternatively, the state can reflect theaccumulation of fatigue damage of the presence of a crack within thetest article. In another embodiment, the state of the article may beinferred from state-sensitive material layers. These layers may beplaced on the surface of the article or embedded within layers of thearticle. In one embodiment, the state-sensitive material can be splitinto strips or some other geometry that facilitates a rapid inspectionor indication of state changes. Preferably, the strips have differentorientations with depth into the test article so that the depth of thestate change can also be readily determined. In a further embodiment,these methods are used to remotely monitor the state or properties of ahidden material so that other material layers are present between thesensor and the material layer of interest.

In one embodiment for monitoring the properties of the materials, thesensors can be eddy current sensors. In one embodiment, the sensors haveseparate layers for the drive winding and sense elements. In anotherembodiment, the conductors for the drive and sense elements are placedin different layers on the test article itself. In another embodiment,the sensor is an array of eddy current sensors. These sensors can bemounted to the surface of the material or scanned over the surface ofthe article. In another embodiment, the sensor is a dielectric sensor.In another, it is a GMR sensor. These sensors can also be protected fromenvironmental damage by durable layers. Suitable durable materialsdepend upon the type of sensor and the environment. In one embodiment,the durable medium is a ceramic. In another, it is a stainless steel.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theembodiment of the invention will be apparent from the following moreparticular description of preferred embodiments of the invention, asillustrated in the accompanying drawings in which like referencecharacters refer to the same parts throughout the different views. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating the principles of the embodiment of the invention.

FIG. 1 is a drawing of a spatially periodic field eddy-current sensor.

FIG. 2 is an expanded view of the drive and sense elements for aneddy-current array having offset rows of sensing elements.

FIG. 3 is an expanded view of the drive and sense elements for aneddy-current array having a single row of sensing elements.

FIG. 4 is an expanded view of an eddy-current array where the locationsof the sensing elements along the array are staggered.

FIG. 5 is an expanded view of an eddy current array with a singlerectangular loop drive winding and a linear row of sense elements on theoutside of the extended portion of the loop.

FIG. 6 shows a representative measurement grid relating the magnitudeand phase of the sensor terminal impedance to the lift-off and magneticpermeability.

FIG. 7 shows a representative measurement grid relating the magnitudeand phase of the sensor terminal impedance to the lift-off andelectrical conductivity.

FIG. 8 shows a plot of MWM measured permeability scans along the axis ofa 4340 steel tensile specimen containing semicircular notches, at twolevels of applied stress. The distance along the scan is in inches.

FIG. 9 shows a plot of MWM measured permeability for a five load-unloadsequence.

FIG. 10 shows a plot of MWM measured permeability for the loads achievedon the increasing and decreasing portions of a load-unload sequence.

FIG. 11 shows the MWM measured magnetic permeability versus bendingstress in a high-strength steel specimen at stresses from −700 to 700MPa. The specimen was shot peened.

FIG. 12 shows MWM measured transverse permeability changes atincrementally increasing and decreasing tensile load (maximum load=53.4kN (12,000 lbs); increment=8.9 kN (2,000 lbs)).

FIG. 13 shows a plot of MWM measured transverse permeability changes fora cyclically changing tensile load.

FIG. 14 shows a plot of normalized permeability against the number offatigue cycles for a shot peened 4340 steel specimen.

FIG. 15 shows an image of the MWM measured permeability of the fatiguedamage zone at the end of the fatigue test.

FIG. 16 shows the effective permeability variation with applied stressfor coated and uncoated samples.

FIG. 17 shows MWM-Array measured permeability changes during thecomplete loading/hydrogen exposure/unloading test cycle.

FIG. 18 shows an illustration of a lap joint with a stress-sensitivematerial and a sensor array.

FIG. 19 shows an illustration of a non-contact measurement of astress-sensitive material.

FIG. 20 shows a layout for a single turn Cartesian geometry GMRmagnetomer.

FIG. 21 shows a layout for a multi-element GMR array.

FIG. 22 shows a schematic for remotely monitoring the temperature of aplate.

FIG. 23 shows the top plate conductivity as a function of temperaturewith and without compensation for changes in the conductivity of thebottom plate, which is between the top plate and the sensor.

FIG. 24 shows a plot of the top plate relative permeability as afunction of the top plate strain at varying levels of applied stress.

FIG. 25 shows the layout of a GMR sensor and 6.4 mm thick sample platefor simulating material loss.

FIG. 26 shows an image of material loss in a 6.4 mm (0.25-in.) thickaluminum plate, generated with the GMR probe at 100 Hz. The four regionsrepresent 3%, 5%, 10%, and 20% loss. Distances and thickness estimatesare in millimeters.

FIG. 27 shows the normalized thickness estimate for two lengthwise scansthrough the centers of the 10% and 3% regions, and the 20% and 5%regions.

FIG. 28 shows the MWM measured conductivity changes for Al 2024 attemperatures up to 270° C.

FIG. 29 shows the temperature and conductivity history for an Al 2024coupon heat treatment.

FIG. 30 shows the MWM measured conductivity transient of Al 7075 duringretrogression.

FIG. 31 shows a normalized permeability image obtained from an MWM-Arrayscanned over a double-notched 4340 low-alloy steel tensile specimenafter failure in a tension test, with an excitation frequency of 1 MHzand the extended portions of the primary winding oriented parallel tothe loading axis.

FIG. 32 shows a normalized permeability image obtained from an MWM-Arrayscanned over a double-notched 4340 low-alloy steel tensile specimenafter failure in a tension test, with an excitation frequency of 158 kHzand the extended portions of the primary winding oriented parallel tothe loading axis.

FIG. 33 shows a normalized permeability image obtained from an MWM-Arrayscanned over a double-notched 4340 low-alloy steel tensile specimenafter failure in a tension test, with an excitation frequency of 1 MHzand the extended portions of the primary winding oriented perpendicularto the loading axis.

FIG. 34 shows an illustration of segments of state-sensitive materialembedded between layers of a test article.

FIG. 35 is a representative single wavelength interdigitated electrodedielectrometer with spatially periodic driven and sensing electrodes ofwavelength λ that can measure dielectric properties of the adjacentmaterial.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

The use of conformable and nonconformable eddy-current sensors andsensor arrays is described herein for the nondestructivecharacterization of materials, particularly as it applies to thecharacterization of applied and residual stresses. This includes surfacemounted and scanning, contact and non contact configuration. Thissensing approach can be used to monitor the material characteristics ata given location with single or multiple sensing element sensors, sensorarrays and/or networks of surface mounted sensors using hand-heldprobes, mounted into automated scanners or as part of an embeddednetwork. The sensors can be mounted into a structure in proximity to amaterial under test for monitoring the property changes while thematerial is being stressed and fatigued. Alternatively, such embeddedsensors can be queried with instrumentation on a scheduled orunscheduled basis with either no electronics on board or minimalelectronics on board, and by plugging in at an easy access location. Thesensors can also be used to detect process related changes in thematerial properties, such as grinding burns in steels either as a partof in-process monitoring or at any time after processing, i.e., duringquality control inspections or in service.

Aspects of this embodiment of the invention address measurement ofdamage, conditions (e.g., from manufacture, rework or repair), stressesmicrostructure changes and other material properties through scanning incontact on non-contact surface mounting, non contact mounting and evenmonitoring one layer or on internal material property through anotherlayer or external material closer to the sensor. Applications includebut are not limited to test materials, coupons, components or systems,bridges, aircraft (e.g., landing gear), towers, construction equipment,nuclear reactor nozzles (e.g., I.D. or O.D., or J-welds), submarines(e.g., temper embrittlement near welds) automobile components, tires(e.g., internal steel components), medical devices or implants, andweapon casings.

A conformable eddy-current sensor suitable for these measurements, theMeandering Winding Magnetometer (MWM®), is described in U.S. Pat. Nos.5,015,951, 5,453,689, and 5,793,206. The MWM is a “planar,” conformableeddy-current sensor that was designed to support quantitative andautonomous data interpretation methods. These methods, called gridmeasurement methods, permit crack detection on curved surfaces withoutthe use of crack standards, and provide quantitative images of absoluteelectrical properties (conductivity and permeability) and coatingthickness without requiring field reference standards (i.e., calibrationis performed in “air,” away from conducting surfaces). MWM sensors andMWM-Arrays can be used for a number of applications, including fatiguemonitoring and inspection of structural components for detection offlaws, degradation and microstructural variations as well as forcharacterization of coatings and process-induced surface layers.Characteristics of these sensors and sensor arrays include directionalmulti-frequency magnetic permeability or electrical conductivitymeasurements over a wide range of frequencies, e.g., from 250 Hz to 40MHz with the same MWM sensor or MWM-Array, high-resolution imaging ofmeasured permeability or conductivity, rapid permeability orconductivity measurements with or without a contact with the surface,and a measurement capability on complex surfaces with a hand-held probeor with an automated scanner. This allows the assessment of applied andresidual stresses as well as permeability variations in a componentintroduced from processes such as grinding operations.

FIG. 1 illustrates the basic geometry of an the MWM sensor 16, adetailed description of which is given in U.S. Pat. Nos. 5,453,689,5,793,206, and 6,188,218 and U.S. patent application Ser. Nos.09/666,879 and 09/666,524, both filed on Sep. 20, 2000, the entireteachings of which are incorporated herein by reference. The sensorincludes a primary winding 10 having extended portions for creating themagnetic field and secondary windings 12 within the primary winding forsensing the response. The primary winding is fabricated in a spatiallyperiodic pattern with the dimension of the spatial periodicity termedthe spatial wavelength λ. A current is applied to the primary winding tocreate a magnetic field and the response of the MUT to the magneticfield is determined through the voltage measured at the terminals of thesecondary windings. This geometry creates a magnetic field distributionsimilar to that of a single meandering winding. A single element sensorhas all of the sensing elements connected together. The magnetic vectorpotential produced by the current in the primary can be accuratelymodeled as a Fourier series summation of spatial sinusoids, with thedominant mode having the spatial wavelength λ. For an MWM-Array, theresponses from individual or combinations of the secondary windings canbe used to provide a plurality of sense signals for a single primarywinding construct as described in U.S. Pat. No. 5,793,206 and Re.36,986.

In another embodiment, eddy-current sensor arrays comprised of one ormore drive windings, (possibly a single rectangle) and multiple sensingelements are used to inspect the test material. Example sensor arraysare shown in FIG. 2 through FIG. 4 some embodiments of which aredescribed in detail in U.S. patent application Ser. Nos. 10/102,620,filed Mar. 19, 2002, and 10/010,062, filed Mar. 13, 2001, the entireteachings of which are incorporated herein by reference. These arraysinclude a primary winding 70 having extended portions for creating themagnetic field and a plurality of secondary elements 76 within theprimary winding for sensing the response to the MUT. The secondaryelements are pulled back from the connecting portions of the primarywinding to minimize end effect coupling of the magnetic field. Dummyelements 74 can be placed between the meanders of the primary tomaintain the symmetry of the magnetic field, as described in U.S. Pat.No. 6,188,218. When the sensor is scanned across a part or when a crackpropagates across the sensor, perpendicular to the extended portions ofthe primary winding, secondary elements 72 in a primary winding loopadjacent to the first array of sense elements 76 provide a complementarymeasurement of the part properties. These arrays of secondary elements72 can be aligned with the first array of elements 76 so that images ofthe material properties will be duplicated by the second array(improving signal-to-noise through combining the responses or providingsensitivity on opposite sides of a feature such as a fastener asdescribed in—U.S. patent application Ser. Nos. 10/102,620 and10/010,062. Alternatively, to provide complete coverage when the sensoris scanned across a part the sensing elements, can be offset along thelength of the primary loop or when a crack propagates across the sensor,perpendicular to the extended portions of the primary winding, asillustrated in FIG. 2.

The dimensions for the sensor array geometry and the placement of thesensing elements can be adjusted to improve sensitivity for a specificinspection. For example, the effective spatial wavelength or four timesthe distance 80 between the central conductors 71 and the sensingelements 72 can be altered to adjust the sensitivity of a measurementfor a particular inspection. For the sensor array of FIG. 2, thedistance 80 between the secondary elements 72 and the central conductors71 is smaller than the distance 81 between the sensing elements 72 andthe return conductor 91. An optimum response can be determined withmodels, empirically, or with some combination of the two. An example ofa modified sensor design is shown FIG. 3. In this sensor array, all ofthe sensing elements 76 are on one side of the central drive windings71. The size of the sensing elements and the gap distance 80 to thecentral drive windings 71 are the same as in the sensor array of FIG. 2.However, the distance 81 to the return of the drive winding has beenincreased, as has the drive winding width to accommodate the additionalelements in the single row of elements. Increasing the distance to thereturn reduces the size of the response when the return crosses afeature of interest such as a crack. Another example of a modifieddesign is shown in FIG. 4. Here, most of the sensing elements 76 arelocated in a single row to provide the basic image of the materialproperties. A small number of sensing elements 72 are offset from thisrow to create a higher image resolution in a specific location. Othersensing elements are distant from the main grouping of sensing elementsat the center of the drive windings to measure relatively distantmaterial properties, such as the base material properties for plates ata lap joint or a weld.

In one embodiment, the number of conductors used in the primary windingcan be reduced further so that a single rectangular drive is used. Asshown in FIG. 5, a single loop having extended portions is used for theprimary winding. A row of sensing elements 75 is placed on the outsideof one of the extended portions. This is similar to designs described inU.S. Pat. No. 5,453,689 where the effective wavelength of the dominantspatial field mode is related to the spacing between the drive windingand sensing elements. This spacing can be varied to change the depth ofsensitivity to properties and defects. In one embodiment this distanceis optimized using models to maximize sensitivity to a feature ofinterest such as a buried crack or stress at a specific depth.Advantages of the design in FIG. 42 include a narrow drive and sensestructure that allows measurements close to material edges andnon-crossing conductor pathways so that a single layer design can beused with all of the conductors in the sensing region in the same plane.The width of the conductor 91 farthest from the sensing elements can bemade wider in order to reduce an ohmic heating from large currents beingdriven through the drive winding. In another embodiment additional rowsof sense elements can be placed on the opposite side of the drive 71 atthe same or different distances from the drive. In another embodimentsensing elements can be placed in different layers to provide multiplelift-offs at the same or different positions.

The MWM sensor and sensor array structure can be produced usingmicro-fabrication techniques typically employed in integrated circuitand flexible circuit manufacture. This results in highly reliable andhighly repeatable (i.e., essentially identical) sensors, which hasinherent advantages over the coils used in conventional eddy-currentsensors. As indicated by Auld and Moulder, for conventional eddy-currentsensors “nominally identical probes have been found to give signals thatdiffer by as much as 35%, even though the probe inductances wereidentical to better than 2%” [Auld, 1999]. This lack of reproducibilitywith conventional coils introduces severe requirements for calibrationof the sensors (e.g., matched sensor/calibration block sets). Incontrast, duplicate MWM sensor tips have nearly identical magnetic fielddistributions around the windings as standard micro-fabrication(etching) techniques have both high spatial reproducibility andresolution. As the sensor was also designed to produce a spatiallyperiodic magnetic field in the MUT, the sensor response can beaccurately modeled which dramatically reduces calibration requirements.For example, calibration in air can be used to measure an absoluteelectrical conductivity without calibration standards, which makes thesensor geometry well-suited to surface mounted or embedded applicationswhere calibration requirements will be necessarily relaxed.

For applications at temperatures up to 120° C. (250° F.), the windingsare typically mounted on a thin and flexible substrate, producing aconformable sensor. A higher temperature version has shown a goodperformance up to about 270° C. (520° F.). In another embodiment thesesensors might be fabricated on ceramic substrates or with platinum leadsand Boron Nitrite coatings or other means to extend their operatingtemperature range. The sensors, which are produced by microfabricationtechniques, are essentially identical resulting in highly reliable andhighly repeatable performance with inherent advantages over the coilsused in conventional eddy-current sensors providing both high spatialreproducibility and resolution. For conformable sensors, the insulatinglayers can be a flexible material such as Kapton™, a polyimide availablefrom E. I. DuPont de Nemours Company, while for high temperatureapplications the insulating layers can be a ceramic such as alumina.

For measuring the response of the individual sensing elements in anarray, multiplexing between the elements can be performed. However, thiscan significantly reduce the data acquisition rate so a more preferablyapproach is to use an impedance measurement architecture thateffectively allows the acquisition of data from all of the senseelements in parallel. Furthermore, ability to measure the MUT propertiesat multiple frequencies extends the capability of the inspection tobetter characterize the material and/or geometric properties underinvestigation. This type of instrument is described in detail in U.S.patent application Ser. No. 10/155,887, filed May 23, 2002, the entireteachings of which are incorporated herein by reference. The use ofmultiple sensing elements with one meandering drive and parallelarchitecture measurement instrumentation then permits high imageresolution in real-time and sensitivity with relatively deep penetrationof fields into MUT.

An efficient method for converting the response of the MWM sensor intomaterial or geometric properties is to use grid measurement methods.These methods map the magnitude and phase of the sensor impedance intothe properties to be determined and provide for a real-time measurementcapability. The measurement grids are two-dimensional databases that canbe visualized as “grids” that relate two measured parameters to twounknowns, such as the magnetic permeability (or electrical conductivity)and lift-off (where lift-off is defined as the proximity of the MUT tothe plane of the MWM windings). For the characterization of coatings orsurface layer properties, three- (or more)-dimensional versions of themeasurement grids called lattices and hypercubes, respectively, can beused. Alternatively, the surface layer parameters can be determined fromnumerical algorithms that minimize the least-squares error between themeasurements and the predicted responses from the sensor, or byintelligent interpolation search methods within the grids, lattices orhypercubes.

An advantage of the measurement grid method is that it allows forreal-time measurements of the absolute electrical properties of thematerial and geometric parameters of interest. The database of thesensor responses can be generated prior to the data acquisition on thepart itself, so that only table lookup and interpolation operations,which are relatively fast, needs to be performed. Furthermore, grids canbe generated for the individual elements in an array so that eachindividual element can be lift-off compensated to provide absoluteproperty measurements, such as the electrical conductivity. This againreduces the need for extensive calibration standards. In contrast,conventional eddy-current methods that use empirical correlation tablesthat relate the amplitude and phase of a lift-off compensated signal toparameters or properties of interest, such as crack size or hardness,require extensive calibrations using standards and instrumentpreparation.

For ferromagnetic materials, such as most steels, a measurement gridprovides conversion of raw data to magnetic permeability and lift-off. Arepresentative measurement grid for ferromagnetic materials (e.g.,carbon and alloy steels) is illustrated in FIG. 6. A representativemeasurement grid for a low-conductivity nonmagnetic alloy (e.g.,titanium alloys, some superalloys, and austenitic stainless steels) isillustrated in FIG. 7. For coated materials, such as cadmium and cadmiumalloys on steels, the properties of the coatings can be incorporatedinto the model response for the sensor so that the measurement gridaccurately reflects, for example, the permeability variations ofsubstrate material with stress and the lift-off. Lattices and hypercubescan be used to include variations in coating properties (thickness,conductivity, permeability), over the imaging region of interest. In oneembodiment, the variation in the coating is corrected at each point inthe image to improve the measurement of permeability in the substratefor the purpose of imaging stresses.

Robust directional magnetic permeability measurements by MWM sensors andMWM-Arrays with grid methods allow estimation of stresses by takingadvantage of the magnetostriction effect. For steels, at magnetic fieldstypical of those used for MWM, the magnetostriction coefficientgenerally is positive, so that the magnetic permeability increases withstress. Thus, once a correlation between stress and MWM measuredmagnetic permeability is established, stresses can be estimated as longas baseline information is available. In another embodiment, bias fieldsor DC offsets in the drive current (possibly using a multiple turn woundor etched drive winding) can be used to move up the B-H curve away fromthe zero field location to improve performance.

An example of a permeability measurement scan with a single elementsensor over a 4340 steel dogbone specimen with semicircular notchesinstalled in a 90 kN (20,000-lb) Instron frame is shown in FIG. 8. Inthis case, the highest stress is expected at the 1.75-in. position withthe highest estimated nominal stress in the narrow section between thesemicircular notches at 16 and 32 ksi, respectively. Magneticpermeability measurements were performed prior to each loading sequence,i.e., at no load and at various levels of tensile load in an incrementalload-unload sequence. The results shown in FIG. 8 were obtained at afrequency of 1 MHz. Multiple frequency MWM measurements can provideinformation on stress distribution with depth. FIG. 9 shows permeabilitychanges in five load-unload sequences to a maximum estimated nominalstress of 8, 24, 32, 48, and 49 ksi. The pattern of the magneticpermeability changes actually reflects the loading pattern. Thepermeability-load curves shown in FIG. 10 illustrates a hysteresisbetween permeability measured at loads achieved on the increasing anddecreasing portions of a loading sequence. This hysteresis is caused bya “delay” in rotation of magnetic domains on unloading.

MWM permeability measurements on 300M high-strength steel specimensunder fully reversed bending loading provide further indication of thecapability of MWM sensors to perform stress measurements. The tests wereperformed on flat shot-peened specimens installed in a bending fixture.The stress range used in the test was between −700 MPa in compressionand 700 MPa in tension. The stresses were determined from strainsmeasured with a BLH strain gage using BLH instrumentation. The straingages were attached to the “back” side. MWM magnetic permeabilitymeasurements were performed with the longer segments of the MWM drivewinding perpendicular to the bending stress direction. In thisorientation, the MWM measures permeability in the specimen longitudinaldirection. FIG. 11 shows how the permeability measured at frequencies of40 kHz, 100 kHz, and 1 MHz changes with applied bending stress. The dataillustrate the sensitivity and quality of the permeability measurementsfor stress measurements in high strength steels over a wide range ofstresses. The results clearly show the sensitivity of the MWMmeasurements to stress changes and reasonably small hysteresis,particularly in the compressive stress range.

The capability to perform directional permeability measurements allowscharacterization of both uniaxial and biaxial stresses, as described forexample in U.S. patent application Ser. No. 10/351,978, filed Jan. 24,2003, the entire teachings of which are incorporated herein byreference. In the latter case, the MWM permeability measurements atvarious sensor orientations reveal the directions of the principalstresses. Furthermore, permeability data from multifrequency MWMmeasurements can be used for reconstruction of stress distribution withdepth. For typical excitation frequencies in the several kHz to severalMHz range, the depth of penetration of the magnetic field is limited toa fairly thin layer near the surface, e.g., the first 0.5 mm (0.02 in.).However, lowering the excitation frequency, for example down to severalHz, and using alternative sensing elements such as magnetoresistive orgiant magnetoresistive sensors, as described for example in U.S. patentapplication Ser. No. 10/045,650, filed Nov. 8, 2001, the entireteachings of which are incorporated herein by reference, permitsmeasurements to a significantly greater depth. Also, MWM-Arrays allowimaging of stress distributions over wide areas.

The single layer designs of the drive and sensing elements supports lowcost fabrication without introducing excessive requirements to alignmultiple layers. This significantly reduces manufacturing costs andincreases the number of suppliers that can fabricate the sensors.However, to obtain reasonable signal to noise levels for such singleturn coils (simple rectangles) at low frequencies, it is necessary toapply more current than is typical for conventional eddy currentsensors, e.g., 1 A. One practical limitation on the sensing element sizeis fabrication costs (e.g., 75 μm line widths and larger are low costwith many suppliers, while smaller line widths is more costly and limitsavailable suppliers). Another limitation is the relative contribution tothe signal of the flux coupled by the active sensing area to the fluxcoupled by the relatively long leads. Thus, these leads are kept closetogether and the novel “flux cancellation” design is used to literallycancel the contribution from these long leads (thus instead of twoconductors entering each sensing element, there are actually fourconductors—two to sense the flux linked by the sensing elements and theleads themselves, and the other two to cancel the contribution from theleads, leaving just the response of the sensing elements).

FIG. 12 shows the results of another set of tests illustrating themagnetic permeability changes due to the Poisson's effect or thetransverse contraction under tensile axial load. A 7-channel MWM-Arraywas mounted on the specimen with the longer segments of the MWM-Arraydrive oriented along the specimen axis, i.e., parallel to tensile loadorientation during tests, so that the magnetic permeability in thetransverse direction is measured. In this test, the tensile load wasfirst incrementally increased by 8.9 kN (2,000 lbs) to the maximumtensile load of 53.4 kN (12,000 lbs) and then incrementally decreased to0. The estimated maximum axial stress in the center of the area wasabout 700 MPa (100 ksi). After each load increment, a constant load wasmaintained for a period of time. The loading pattern and MWM-Arraymeasured transverse permeability in all seven channels is shown in FIG.12. The observed change in MWM-Array measured transverse permeabilityappears to mimic changes in transverse strain. The lowest permeabilitychanges occur near the center. The results emphasize the importance ofpermeability measurements and suggest that bidirectional permeabilitymeasurements are critical to stress measurements even under uniaxialloading. Similar results are obtained with the cyclic loading pattern ofFIG. 13, which had a mean load of 8,000 lbs and load amplitudeprogressively increasing from 1,000 lbs (load range of 2,000 lbs) to4,000 lbs (load range of 8,000 lbs).

The ability to detect and image stress distributions has implicationsfor the detection and imaging of early stage fatigue damage as well.Fatigue tests of 4340 steel specimens revealed the capability to detectprecrack damage early in the fatigue life. These specimens were designedwith a cylindrical cavity in the gage section, where an MWM-Array couldbe mounted, and reinforcement ribs on the back side. This provides anonuniform stress distribution with the maximum stress in the centralportion of the cavity, as verified by a finite element analysis, beneaththe footprint of the MWM-Array. The shape and stress distribution withinthe cylindrical cavity can be varied to simulate the geometry of highstrength steel components of interest. In one embodiment MWM orMWM-Array sensors are oriented with their longer winding segmentsaligned parallel or perpendicular to the direction of likely fatiguecrack orientation. The sensor aligned perpendicular to this direction ismost sensitive to fatigue damage and crack monitoring, while the sensorwith longer drive segments parallel to this direction is most sensitiveto stress (i.e., magnetic permeability is measured dominantly in thedirection perpendicular to the longer drive segments, whileconductivity, or induced current flow, is sensed dominantly parallel tothe direction or the longer winding segments). In another embodiment,multiple series connected or multiplexed eddy current sensors, such asMWM-Arrays, are mounted at selected critical and non critical locationsto support both fatigue and stress monitoring either continuously orperiodically or on a scheduled or unscheduled basis depending onconvenience or loading/fatigue/overload events.

In another embodiment, MWM-Arrays we used in a surface mounted or evennon contact (where lift-off is measured using grid methods) to monitorstress and proximity (or vibrations). As with strain gages orextensometers this information can be used to control load frames,monitor changes in material properties or structures, or monitor inservice behavior and damage. Integration of information with that fromstrain gages or extensometers can be used to support decisions regardingfitness for service, material life or to assess material performance infatigue tests. Combination with new fiber optic strain gages is alsouseful.

FIG. 14 shows the permeability changes during a test using a 7-channelMWM-Array. There is virtually no change in the measured permeability upto 7,000 cycles. The change in the slope of MWM measured permeability inthe four centrally located channels at about 7,000 cycles is most likelyassociated with residual stress relaxation and precrack fatigue damage.This fatigue damage stage extends, perhaps, up to 17,000 cycles followedby initiation and extension of multiple microcracks. Two of the channelsshow a significant permeability increase at 32,000 cycles indicatingcoalescence of closely spaced cracks and faster crack growth. SEManalysis on this specimen revealed a few small cracks, with the longestcrack approximately 200 ÿm (0.008 in.) long. This crack was alsoconfirmed by fluorescent liquid penetrant inspection (FPI). The FPIindication appeared as a tiny “speck” judged to be on the order of0.25-mm (0.01-in.) long.

The fatigue critical area of this specimen was also scanned with animaging MWM-Array, with the drive oriented perpendicular to the axis ofthe coupon cavity. This orientation is perpendicular to anticipatedpredominant orientation of fatigue cracks, and is the same as in fatiguetest monitoring of FIG. 14. FIG. 15 shows a permeability image andaligned intermittent regions of increased permeability having a combinedlength of about 20 mm (0.75-in.) Three of these regions appear tocontain short indications characterized by the highest measuredpermeability. The other relatively high permeability regions are likelyto indicate stress relaxation due to the cyclic loading and fatiguedamage prior to formation of detectable cracks. These regions ofenhanced permeability are also consistent with the higher stress regionof the component from the finite element analysis.

For cadmium-plated high-strength steel components, it is important toaccount for the effect of the cadmium layer on the MWM measurements.This is illustrated in FIG. 16, where a coating model was applied to themultiple frequency data obtained from MWM measurements on the 300M highstrength steel specimens. Qualitatively, this data (from 39.8 kHz to 1MHz) showed a decrease in the effective permeability and lift-offcompared to measurements on the uncoated specimen. This is consistentwith the presence of a nonmagnetic conducting surface layer onmagnetizable substrate. The model assumed a Cd layer (electricalconductivity of 22% IACS, 12.76 MS/m) on top of a magnetizable substrate(electrical conductivity of 3.4% IACS, 2 MS/m), so that the unknowns inthis model were the lift-off, Cd layer thickness, and permeability ofthe substrate (steel). The stress distribution, and hence the magneticpermeability, is not necessarily uniform with depth into the substrateand definitely not uniform for a shot peened steel. As the first step,the thickness of the Cd layer on an unstressed sample was estimatedusing a least-squares minimization routine on the multiple frequencydata. In another embodiment a fast table lookup within a lattice isused. Assuming a substrate permeability of 57.1, the Cd thickness wasestimate to be 1.5 ÿm. Using this thickness, substratepermeability/lift-off grids were then generated so that the effectivepermeability of the substrate could be determined. FIG. 16 showspermeability vs. stress curves for non-plated steel, for Cd-plated steelusing a model that does not account for the Cd layer, and for Cd-platedsteel using a model that does account for the Cd layer. As shown in FIG.16, using grids that have a thin Cd layer can provide estimates of thepermeability that are similar between the coated and uncoated samples.Without this compensation for the presence of the Cd coating, thepermeability estimates are significantly reduced for the coated sample.

The numerical value for the Cd layer thickness of 1.5 μm is smallcompared to the nominal thickness of 10-20 μm because of the assumedconductivity for the layer. For these relatively thin layers andintermediate excitation frequencies, the measurements are essentiallysensitive to the product of the layer thickness and electricalconductivity. For alloy layers (e.g., cadmium-titanium alloys) or formicrostructural variations due, for example, from porosity introducedduring the coating process, the electrical conductivity can be lower, inthe range of 1.2-7.0 MS/m (2-12% IACS) and the corresponding thicknesslarger. The thicker Cd layer values can be accommodated, withoutappreciably affecting the permeability estimates, if a lowerconductivity is used for the Cd layer.

The properties of the coating material layer and even the base materialitself can be obtained from multiple parameter estimation approaches.The use of multiple frequencies allows more than two parameters to beestimated. As an example, three, four and five parameter estimationroutines have been developed for determining the properties of coatings,such as MCrAlY coatings used on turbine blades and vanes. As describedin more detail in the DOE Phase II proposal “Intelligent Probes forEnhanced Non-Destructive Determination of Degradation in Hot-Gas-PathComponents,” the entire teachings of which are incorporated herein byreference, a four parameter estimation routine is used for determiningthe coating electrical conductivity and thickness, the sensor lift-off,and the substrate electrical conductivity for nonmagnetizable materials.A five parameter algorithm that allows determination of an additionalparameter, e.g., magnetic permeability when one of the layers ismagnetizable is also described. Clearly, this multiple parameterestimation approach can be applied to different combinations ofelectrical and geometric properties for the various layers. This alsoapplies to shot peened materials or materials with a degradednear-surface layer. For example, it was successfully demonstrated forturbine blades with two types of coatings. Since the processing timetypically increases as the number of properties to be estimatedincreases, this approach for determining the layer and substrateproperties can be performed prior to continuous stress measurements sothat simpler models or measurement grids can be used while themeasurements are being performed. For example, this can involvecharacterizing the coating thickness and conductivity using the multipleparameter algorithm, using this information to generate substratepermeability-lift-off grids, and then performing the continuous stressmeasurements. Alternatively, rapid table look up in lattices orhypercubes can be performed.

Measurements can also be performed while the hydrogen embrittlementoccurs. As an example, cathodic charging was used to introduce hydrogeninto the steel using a setup similar to some previously described[Grendahl, 2002]. Here, a flat 4340 steel specimen is placed in thebending fixture with a 0.4-liter vessel with a 25 mm hole in the bottommounted and sealed on the top of the 4340 steel specimen. The specimenis loaded to stresses as high as 150 ksi. A strain gage mounted to thecompressive side of the specimen provides an independent measure of thestrain so the bending stresses during loading and cathodic charging maybe monitored. After loading to the desired stress level, thepermeability is measured until an initial transient period passes. Thenapproximately 0.3 liters of a 3.5% NaCl solution is poured into thevessel. An electrochemical cell is formed with the 4340 specimen as theworking electrode. A graphite rod serves as the counter electrode and astandard calomel electrode (SCE) provides a reference point for theelectrochemical potential. The working electrode, counter electrode, andSCE were connected to a Schlumberger SI 1286 potentiostat and a selectedpotential between −1.15 V and −1.25 V was applied to effect cathodiccharging. Magnetic permeability measurements with an MWM-Array can beperformed in either scanning mode or with a permanently mountedMWM-Array, on the exposed (tension) side of the test specimen. This typeof in-process monitoring can be performed for other processes thatinclude material property changes.

FIG. 17 shows MWM-Array measured permeability changes during thecomplete test cycle for one specimen. The normalized permeabilitymeasurements were made with the 7-channel MWM-Array attached on thetension side of the specimen. In this test, the specimen was firstmonitored with the mounted MWM-Array before a bending load was appliedto the specimen. Then, the specimen was loaded in bending to 700 MPa(100 ksi) stress level as measured with a strain gage. The MWM-Arraymeasured permeability increased due to the stress from point a to pointb. Between points b and c, the specimen was monitored by the MWM-Arrayat the constant stress level. Then, at point c, the vessel was filledwith 3.5% NaCl solution, the initial open circuit potential wasmeasured, adjustments made, and −1.15 V potential (relative to thestandard calomel reference electrode) was applied at point d. After aninitial period of a fairly stable magnetic permeability between sets 120and 160, i.e., for about three hours, a distinct and steady increase inthe MWM-Array measured permeability was observed for the next ninehours, i.e., up to the time when polarization was discontinued (up topoint g). Note that the potential was changed to −1.25 V at point f.Between points g and h, the stress was maintained, while there was noapplied potential. Finally at point h, the specimen unloading started.

Example sensor arrays are the MWM-Arrays shown in FIG. 2 through FIG. 5,although other array formats can also be used, such as those describedin U.S. patent application Ser. Nos. 09/666,879 and 09/666,524. Theseapplications also describe using a magnetic material in combination withthe sensor as a load cell and adjusting the sensitivity of the responsethrough the selection of the type and dimensions of the material.Alternatively, if the changes in the stress distribution occurrelatively slowly, periodic measurement of the stress distribution canbe performed. This can be accomplished with occasional measurements withan MWM-Array that has been mounted to a surface or by scanningeddy-current sensing arrays over the surface to provide a completemapping of the material properties over the entire surface. In addition,measurements in multiple orientations, preferably two orthogonalorientations, can be performed to determine anisotropic materialproperty variations associated with changes in stresses in the fastenersand can be determined with directional eddy-current sensor arrays. TheMWM-Array is one such example as the sensing elements respondpreferentially to the magnetic permeability oriented perpendicular tothe extended segments comprising the primary winding. In anotherembodiment the larger drive segments are oriented at an angle, e.g., 45degrees, relative to the scan direction. If cracks are likely to form inthe direction perpendicular to the scan direction, then this isnecessary to increase sensitivity to crack size or precrack damage.

In another embodiment an array is configured in an x or v so that twodrive orientations are provided in as two or one layer respectively. Anexample of this configuration is described in U.S. patent applicationSer. No. 10/419,702, filed Apr. 18, 2003, the entire teachings of whichare incorporated herein by reference. In one embodiment the legs of thev or x are at different angles relative to the scan direction (or the xor v is rotated relative to the scan direction) so that cracks responddifferently and material variations that are isotropic respond the same.This improves discrimination capability. This can also be used forburied object imaging.

Conventional eddy-current designs are not ideal for permanent mounting.Conventional eddy-current techniques require varying the proximity ofthe sensor (or lift-off) to the test material or reference part byrocking the sensor back and forth or scanning across a surface toconfigure the equipment settings and display. For example, for crackdetection the lift-off variations are generally displayed as ahorizontal line, running from right to left, so that cracks or othermaterial property variations appear on the vertical axis. Affixing ormounting the sensors against a test surface precludes this calibrationroutine. The probe-to-probe variability of conventional eddy-currentsensors prevents calibrating with one sensor and then reconnecting theinstrumentation to a second (e.g., mounted) sensor for the test materialmeasurements. These shortcomings are overcome with conformableeddy-current sensors that provide absolute property measurements and arereproduced reliably using micro-fabrication techniques. Calibrations canalso be performed with duplicate spatially periodic field sensors usingthe response in air or on reference parts, which may simply be differentareas of the same component, prior to making the connection with thesurface mounted sensor. The capability to characterize fatigue damage instructural materials, along with the continuous monitoring of crackinitiation and growth, has been demonstrated, as described in U.S.patent application Ser. Nos. 09/666,879, 09/666,524, and 10/102,620.This inspection capability is suitable for on-line fatigue tests forcoupons and complex components, as well as for monitoring ofdifficult-to-access locations on both military and commercial aircraft.In another embodiment, mounted sensors are removed to perform otherexaminations (e.g., acetate replicas to defect small cracks), and mustbe reattached using a reference calibration to the previously recordedvalue. By recalibrating the sensor with the same values as beforeremoval, then the continuation of monitoring will begin with the samevalues, e.g., permeability, as before the sensor was removed andreinstalled.

Another aspect of this embodiment of the invention is the inspection ofnickel alloy engine materials, such as Alloy 738 or Alloy 718, whereshot peening and/or heat treatment may produce near surface relativepermeability greater than 1.0. This is also described in U.S. patentapplication Ser. No. 10/419,702. Higher permeability regions aretypically created near the surface by the shot peening and/or heattreatment process. At sufficiently high frequencies, the magnetic fieldis confined near the surface of the MUT and reflects only thepermeability (and stress) of the surface region. At lower frequencies,the magnetic field can penetrate through this region and the average oreffective permeability is reduced. At sufficiently low frequencies, themagnetic field penetrates far enough into the base material that thepermeability approaches 1.0. High resolution images of permeability canthen be used to map residual stress variations to qualify shot peeningor other manufacturing processes or to assess material aging/materialdegradation, as described in more detail in U.S. patent application Ser.No. 10/351,978. Then, regions with unacceptable residual stresses mightbe reworked (e.g., blending and reshot peening) to extend life.

Another aspect of this embodiment of the invention relates to theapplication of a stress-sensitive material to a test material andmonitoring the properties of this stress-sensitive material to infer thestress distribution or mechanical load on the test article. The stresssensitive material could be a magnetic material in which the magneticpermeability changes significantly with stress, as illustrated in FIG.11. An alternative stress-sensitive material is one whose electricalconductivity changes significantly with stress. Also, different layerswith different sensitivity might be used, for example some layers withpermeability being sensitive, and other layers with conductivity beingsensitive. These materials could be magnetic or nonmagnetic. In general,according to the literature on strain gages, metals typically have agage factor reflecting change in resistance per unit strain of between 2and 4. Representative values are listed in Table 1. Preferable materialsfor nonmagnetic stress-sensitive materials are platinum and platinumalloys because of the relatively large gage factors. It should be notedthat conductivity variation with strain tends to become nonlinear forlarge strains and the listed gage factors are most applicable tosituations of low strains. The choice of the stress-sensitive materialcan therefore depend on the strains anticipated for the inspection.

TABLE 1 Gage factors for stress-sensitive conducting materials. MaterialComposition Gage Factor Platinum 100% Pt 6.1 Platinum-Iridium 95% Pt, 5%Ir 5.1 Platinum-Tungsten 92% Pt, 8% W 4.0 Isoelastic 55.5% Fe, 36% Ni,8% Cr, 0.5% Mo 3.6 Karma 74% Ni, 20% Cr, 3% Al, 3% Fe 2.4 Constantan 55%Cu, 45% Ni 2.0 Nichrome 80% Ni, 20% Cu 2.0 Monel 67% Ni, 33% Cu 1.9Manganin 84% Cu, 12% Mn, 4% Ni 0.47 Nickel 100% Ni −12.1

Monitoring the properties of a stress-sensitive material attached to atest material is most useful in situations where direct nondestructivemeasurements of the stresses in the test material are relativelydifficult, such as in aluminum with eddy-current sensors. In contrast,monitoring the permeability changes of a layer of magnetic material orelectrical conductivity changes of a layer of stress-sensitivenon-magnetic material integrally attached to the test article can offersubstantially greater sensitivity. The properties of the attached layermaterial can be monitored using a permanently mounted sensor or with ascanning sensor array to create images of the stress distribution. Anillustration of this approach is given in FIG. 18, where thestress-sensitive material 130 is affixed to the back of the lap joint100, which has layers 104 and 106 joined by fasteners 108. The sensingelements 122 are shown in a linear array, but they could also bedistributed among and around the fasteners as well. In this case, thedrive winding is not shown. The measurements can also be performed in anon-contact fashion, as shown in FIG. 19, where an air gap 134 ismaintained between the sensor or sensor array 136 and the test material132. In both FIG. 18 and FIG. 19, the magnetic fields generated by theeddy-current sensor are projected through the test material so that theremote fields interact with the attached stress-sensitive layer and thesensor and attached layer material effectively operate as a load cell.In one embodiment, stress sensitive materials (or materials moresensitive to other affects such as temperature) might be located on thenear and far side of a layer or at multiple depths within a multiplelayered construct. Also, as described latex strips, composites or otherdirectionally sensitive media might be used to measure stress indifferent directions or to simply aid in differentiating between affectsat different depths.

The sensitivity of this measurement approach is affected by theelectrical and geometric properties of the stress-sensitive layerattached to the test material. The material should be selected so thatthe permeability or conductivity change for an anticipated stress levelis detectable with the sensor and instrumentation. Furthermore, thematerial should be relatively thin to better reflect the stressdistribution of the test material. However, it should also be thickenough to provide a measurable signal with the sensor or sensor array.Selection of the thickness of the layer must therefore balance thesecompeting effects. The magnetic or non-magnetic stress-sensitivematerial can also be applied to the surface of the test material nearthe sensor.

The properties of the stress-sensitive material and even the basematerial itself that the coating is applied to can be obtained frommultiple parameter estimation approaches. The use of multiplefrequencies allows more than two parameters to be estimated. As anexample, three, four and five parameter estimation routines have beendeveloped for determining the properties of coatings, such as MCrAlYcoatings used on turbine blades and vanes, as described above.

This type of state sensitive material layer could also be applied otherinspection or monitoring applications. Instead of stress, the sensitivelayer may have a greater temperature sensitivity so that the temperatureof the material can be monitored. It may also permit the detection andcharacterization of thermal or mechanical overload events that cancompromise the future use of the article.

As another alternative embodiment, in addition to inductive coils, othertypes of sensing elements, such as Hall effect sensors, magnetoresistivesensors, SQUIDS, and giant magnetoresistive (GMR) sensors, can also beused for the measurements. The use of GMR sensors for characterizationof materials is described in more detail in U.S. patent application Ser.No. 10/045,650. While conventional eddy-current sensors are effective atexamining near surface properties of materials, but have a limitedcapability to examine material property variations deep within amaterial. GMR sensors respond to magnetic fields directly, rather thanthrough an induced response on sensing coils, which permits operation atlow frequencies, even DC, and deep penetration of the magnetic fieldsinto the test material. The GMR sensors can be used in place of sensingcoils, conventional eddy-current drive coils, or sensor arrays. Thus,the GMR-based sensors can be considered an extension of conventionaleddy-current technology that provides a greater depth of sensitivity tohidden features and are not deleteriously affected by the presence ofhidden air gaps or delaminations. In an alternative embodiment electricfield sensors, IDEDs described in U.S. Pat. Nos. 4,814,690 and 6,380,747and in U.S. patent application Ser. Nos. 10/040,797, filed Jan. 7, 2002,and 10/225,406, filed Aug. 20, 2002, the entire teachings of which arehereby incorporated by reference might be used to monitor stress ortemperature, moisture content or contamination or overload of fatigue inadhesives, epoxies, glass, oil, plastics and in single or multiplelayered media. Here the conductivity and dielectric constant or complexpermittivity and layer thicknesses are measured using the same methodsas for magnetic field sensing. In one such electric field methodmultiple layers of material are added on top of a sensor each sensitiveto different chemicals or biological materials. In another embodimentcoatings on pharmaceuticals are characterized for manufactured quality,such as density, moisture content or density of suspended particles in atime release coating.

There are a variety of inspection applications that could take advantageof the capabilities of a GMR-based sensing technology. One is theimaging of damage, such as cracks, inclusions, and corrosion, deep belowthe surface of conducting materials typically found in aging aircraft.These measurements can also be performed on magnetic materials where theskin depth is relatively small when conventional eddy-current technologyand excitation frequencies are used. Since the GMR sensor responds tomagnetic field variation and the magnetic field created by a drive coilor primary winding can readily penetrate air gaps, these sensors can beused for non-contact and remote measurements where air-gaps are presentbetween conducting and/or magnetic material layers. For example,measurements of the electrical properties, such as the electricalconductivity or magnetic permeability, of inaccessible materials can beused with correlation curves to assess the temperature or stress of thehidden materials. As an example, the use of direct temperaturemeasurements for characterizing the thermal response of inaccessiblesurfaces is considered an ill-posed inverse heat conduction problem. Incontrast, low-frequency magnetic fields can penetrate the test materialsto inspect the remote surface with simultaneous measurements at higherfrequencies used to compensate for near-surface property variations.This new capability may be suitable for health monitoring for engines.An example is the monitoring of internal temperatures in aircraftengines, for example, in the compressor or turbine stages. Thistechnology may also be suitable for the non-contact remote monitoring ofstresses in ferrous materials, such as landing gear after a hard landingor for monitoring the stresses on steel bolt in an aluminum structurethrough the aluminum at relatively low frequencies or even at DC or inother steel structures.

GMR-based magnetometers can be incorporated into a variety of primarywinding geometries, such as rectangular, circular, or elliptical coils.An example rectangular or Cartesian-geometry GMR-based magnetometer isillustrated in FIG. 20. A GMR array is shown in FIG. 21. The parallelprimary windings are laid out in a fashion similar to the MWM-Arraydesigns, which has shown advantages in the generation of C-scan images.

The winding layout of FIG. 20 allows the relative polarity of the twoconstituent current loops to be changed to generate a structure withtwice the effective wavelength. This permits inspection at two differentdepths, at a single frequency. For the results presented here, thecurrents were directed as shown, with the center leg of the windingcarrying twice the current of the two edge (or return) windings. The twohalf-wavelength current loops have identical areas and oppositelydirected currents of equal magnitude, so that their effective dipolemoments cancel out, resulting in no net dipole moment in the far field.Here the dominant far-field moment is a quadrupole. This elimination ofthe net dipole moment of the sensor improves measurement reliabilitybecause it reduces the sensitivity of the magnetometer to objectsoutside its nominal range of sensitivity.

The GMR sensing element requires biasing with a constant magnetic field.To provide this bias, and to address the high nonlinearity of thesensor's transfer characteristic, it is placed in a feedbackconfiguration with a secondary biasing coil. In this way, the magneticfield at the GMR sensor remains nearly constant during operation,eliminating the effect of the nonlinear transfer characteristic, whilemaintaining sensitivity at low frequencies. The magnitude of the currentin the secondary winding is taken as the output signal, and since therelationship between this current and the magnetic field for an air-corewinding is perfectly linear, so is the transfer characteristic of theentire hybrid sensor structure.

One example application using a GMR sensor is for monitoring propertiesthrough intermediate layers of metal. In this case, the absoluteelectrical properties are measured through thick metal plates and thenrelated to other physical properties of interest. FIG. 22 shows one suchlayered geometry, with a low frequency (100 Hz) measurement used toremotely monitor the temperature dependent conductivity variation of analuminum plate through a 0.25-in. thick aluminum plate. The thickness ofthe upper plate (remote from the sensor), the conductivity and thicknessof the bottom plate (near the sensor), as well as its lift-off(proximity) from the sensor windings, are incorporated in the model usedto generate the appropriate measurement grids. The two unknownproperties are the conductivity of the upper plate and the thickness ofthe thermally insulating nonconducting spacer between the two plates,which also varied significantly with the temperature of the upper plate.The ability to measure the two unknown parameters independently isdemonstrated by taking measurements at room temperature with spacers ofvarying thickness and demonstrating that the data follow aconstant-conductivity line in the grid. To verify and record the actualplate temperatures, thermocouples were attached to both metal plates.The top plate was initially chilled and then gradually heated with a hotair gun. The data of FIG. 23 shows that both the conductivity and spacerthickness are affected by the plate temperature.

In this experiment, the temperature of the bottom plate also increased,despite the thermal insulation. Ignoring this effect yields the plot inFIG. 23 with cross symbols. To compensate for the temperature variationof the bottom plate, data were also taken at 10 kHz simultaneous withthe 100 Hz measurement. At this higher frequency the bottom plateappears infinitely thick since it is more than several skin depths thickand a simple conductivity/lift-off grid can be used to independentlydetermine the bottom plate's conductivity. Once this value is obtained,it can be used in the estimation of the upper plate conductivity via athree-dimensional measurement grid, called a grid lattice. Using thismethod, the data shown with squares in FIG. 23 are obtained. Asexpected, it follows a linear relationship. Example applications of suchremote temperature and stress measurement include: 1) measuringtemperature of carbon fibers in an epoxy matrix, 2) measuring stress ortemperature on fibers in a composite where the fibers are coated with amagnetic or other material that can be sensed using magnetic or electricfields, 3) measuring temperature on the outside of a wing or othercavity from the inside using sensors mounted on the inside with orwithout an added more sensitive on the outside of the metal (or othermaterial) layers, 4) measuring stress on a bearing race at the outsidediameter (or rolling surface) from the inside diameter or using anembedded sensor, 5) measuring stress or temperature as a function ofdepth, 6) measuring stress or temperature variations with time and/orspace near an internal geometric feature such as a cooling hole or atthe inlet of an aircraft engine.

Another example of demonstrated measurements with a GMR sensor is themonitoring of stress. In this case, measurements were performed on ahidden steel layer in a thick structure. A 1.4 mm thick steel plate wassuspended over a 6.7 mm Al 6061 plate using a 3 mm thick spacer locatedin the center. A 5 kg weight was used to keep the center part of theplate from moving. The measurement grid used in this case was apermeability/spacer thickness grid. The spacer thickness was one of theunknowns since it varied as the steel plate was deformed under theapplied force. Zero stress is registered when the plate is placed on aflat surface. The measured relative permeability as a function of theapplied stress at the bottom of the plate are shown in FIG. 24. Thisillustrates the capability to measure stress (or strain) on a buriedsteel layer through relatively thick intermediate aluminum andinsulating layers.

GMR-based sensors can also be used to measure material loss ofrelatively thick parts. A representative sample is shown in FIG. 25. Inthis case, four flat-bottom circular areas were milled out of a 6.4 mm(0.25-in.) aluminum alloy plate. The depths of the four depressions,simulating material loss, are 3%, 5%, 10%, and 20%, respectively. Usingthe GMR probe and an excitation frequency of 100 Hz, the thickness scanimage in FIG. 26 was generated. The orientation of the sensor probe issuch that the primary windings lie along the vertical direction, withthe secondary positioned in the left half-wavelength. The “double hump”signature produced by this sensor is apparent. The plot in FIG. 27 showsthe normalized thickness estimate for two of the 220 individuallengthwise scans that comprise the area scan, through the centers of the10% and 3% regions, and the 20% and 5% regions. The source of the“double-hump” response to what is a square pulse in thickness change,stems from the fact that the largest change to the signal is observedwhen the perturbation (e.g., thickness change) falls under a primarywinding. The main lobe is generated by the response to the centerwinding, which carries twice the current and is closer to the secondarythan the two outer winding legs. The second smaller lobe is the responseto the outer (or return) primary winding on the opposite side of thesecondary. It has the same polarity, because both the current directionand the relative position with respect to the secondary are reversedwith respect to the center winding. The response to the third leg of theprimary winding is weaker still and appears to the left of the mainlobe. Its polarity is reversed because while the current flows in theopposite direction, it is on the same side of the secondary as thecenter winding.

The edge effect, i.e. when part of the sensor footprint is outside thesample area, is very prominent both in the plot and in the scan image.It is much more pronounced in the horizontal direction, because of thegeometry of the probe, which is built to appear uniform (and inprinciple infinite) in the direction parallel to the longer drivewinding segments. The edge effect is more prominent on the left side,since the secondary sensing element is closer to the left edge of theprobe. The polarity of the edge effect is opposite on the two sides asdescribed above. The vertical edge effect is not present in the scanimage because the image was normalized by the thickness in the areabetween the 140- and 150-mm positions.

The MWM sensors and sensor arrays can also be used to monitor heattreatment of metals and for monitoring high temperature tests on metalsand alloys, metal processing and the condition of elevated temperaturecomponents. FIG. 28 shows the results of standard MWM configurationlift-off and conductivity measurements of an aluminum 2024 coupon duringvarious heat treatments at temperatures up to 270° C. The plots showsconductivity versus temperature for an aluminum 2024 coupon as theoriginal T3 condition was overaged and then reheated. Following asolution anneal to produce the T42 condition, the coupon was againheated to monitor conductivity changes as the T62, T72 and overagedconditions were achieved. These measurements used a single-channel MWMsensor constructed of copper conductors on a Kapton™ substrate. Thesensors can also be fabricated onto ceramic substrates, such as alumina,for even higher temperature operation. Although not generally flexible,these ceramic substrate arrays can be molded into the shape of the testarticle. FIG. 29 shows the corresponding temperature and conductivityhistory, which illustrates the capability of the sensor to monitor theprocessing of the metal and the condition during the treatment.Similarly, FIG. 30 illustrates the alloy condition changes duringretrogression. The conductivity drops dramatically after the coupon isplaced in the furnace. Then, after dwelling at or exposing the coupon toan elevated temperature such as 400° F. the coupon is quenched. Thisresults in a higher electrical conductivity than was present initially,with the effect depending on the exposure duration.

MWM-Arrays also provide a capability to perform bi-directional magneticpermeability measurements in a scanning mode. FIG. 31 through FIG. 33provide images of the magnetic permeability for a broken tensilespecimen of 4340 low alloy steel. The MWM-Array was scanned across andalong the gage section of a specimen broken in a tensile test and thepermeability was measured at two frequencies, 158 kHz for FIG. 32 and 1MHz for FIG. 31 and FIG. 33. In FIG. 31 and FIG. 32 the extendedportions of the primary winding were oriented parallel to the loadingaxis. In FIG. 33 the extended portions of the primary winding wereoriented perpendicular to the loading axis. This illustrates thepotential to map residual stress variations produced in parts fabricatedfrom carbon and low alloy steels, for example by a hard landing in thelatter case. Notice that the permeability images at low and highfrequencies reveal stress changes with distance from the surface. A highresidual stress region near the fracture is indicated in the images ofFIG. 33. To create these images, a permeability/lift-off measurementgrid was used, assuming a known conductivity and an infinite half-space(i.e., the steel layer is assumed to be infinitely thick). Since thelift-off or distance between the sensing windings and the test materialis being measured through the measurement grids, the residual stressmeasurement can be performed in a non-contact mode, which ensures thatthe sensor and probe assembly do not influence the stress distributionon the component.

These eddy current sensors and sensor arrays permit the measurement andmonitoring of stresses (applied and residual) in steel components. Thesensors can be used to inspect selected locations on a part by placingthe sensor over the area of interest, scanning over the area, orpermanently mounting or affixing the sensor to the surface. By measuringwith multiple sensor orientations, the permeability and stressdistribution can be inferred. Preferably, the orientations areperpendicular to one another so that the biaxial stress distribution isobtained. The anisotropy is most easily obtained when the orientation ofthe sensor or sensor array has the direction of greatest sensitivityaligned with the directions of the maximum and minimum principalstresses in the materials. When the sensors are flexible and can conformto the complex geometry surfaces, the sensors can be supported by abottom foam support that makes the sensor essentially flat until placedonto the surface. Alternatively, the sensors can be molded into afixture that conforms to or has a shape similar to the geometry of thetest material.

These same techniques can be used to detect and characterize overloadeffects on components. As examples, excessive mechanical or thermalloading on a component can compromise the structural integrity of acomponent so that it fails during subsequent use. In magnetizablematerials such as most steels, patterns of the magnetic permeabilitydistribution over the surface of critical components can reflect theseoverload conditions. The permeability can be measured using singleelement eddy current sensors placed at selected locations and indifferent orientations, eddy current sensor arrays permanently mountedat selected locations in different orientations, and by imaging sensors,such as MWM-Arrays, scanned in different orientations.

The capability to monitor applied and residual stress on a componentalso permits other properties to be inferred, such as the weight of anarticle. For example, the weight of an aircraft or changes in the weightin the aircraft can be inferred from magnetic permeability measurementperformed on steel landing gear components. Eddy current sensor arrayscan be scanned or mounted at selected locations that reflect the loadtransfer from the weight of the aircraft about the landing gear. Thearrays should be mounted or scanned in the orientation parallel to thedirection of maximum principal stresses. Other orientations could alsobe included to provide a more complete observation of the stressdistribution.

Combinations of permanently mounted and scanning sensors and sensorarrays can be used to image and monitor stress distributions on simpleand complex surfaces. The surfaces can be flat or non-flat geometry andmay be on layered materials, such as lap joints. Measurements can alsobe performed in contact or non-contact modes, with parts of the sensormounted on an opposing surface.

These can be in the form of layers, gradient materials with propertiesvarying with depth or near critical features such as holes, fibers orcoated fibers, weld material or entire components such as a fitting,bolt, or joint. State-sensitive materials can also be used in the designand use of critical components. These materials can be ones whosemagnetic permeability, dielectric permittivity, or electricalconductivity varies with stress, temperature, thermal or mechanicaloverload, fatigue damage, crack presence, or some combination of theseeffects. The materials would be selected for use with a particularcomponent based on the sensitivity of the materials and the propertiesof the component. For example, Table 1 listed several materials suitablefor an eddy current sensor but these same materials would not besuitable for a dielectric sensor.

These state-sensitive materials could be placed on the surface orembedded between material layers of the test component. FIG. 34 shows asensor 160 above the top layer 150, middle layer 152, and bottom layer154 of a three layer component. State-sensitive materials are embeddedbetween the top and middle layers 156 and between the middle and bottomlayers 158. For monitoring stress or fatigue, the state-sensitive layerscan contain laminates of multiple strips or some other pattern that hasdifferent orientations at the different depths in the material. When theproperties of the layers (156 and 158) are measured, the pattern for theeffective property (e.g., permeability) will change depending upon thestate (e.g., stress) on the component. This can also be applied to themonitoring of stress around fasteners, as described in U.S. patentapplication Ser. No. 10/351,978.

In a similar fashion, eddy current sensors can be mounted between layersto measure fatigue damage and stresses or for detection of cracks. Thiscan also include placing the electrical conductors for the drive andsense windings on different layers of the component as well. The sensorsthemselves can also be supported with durable substrates, such asstainless steel or ceramic to prevent damage to the sensor conductors.The sensors may even be embedded within these durable supports. In oneembodiment one or two sets of drive winding with or without a magneticsubstrate and with or without a protective coating to limit frettingdamage are embedded between layers with or without an embedded array ofsensing elements. An array of sense elements is then scanned across theouter surface to measure the field. This increases sensitivity to cracksor stress changes between the layers because the applied field does nothave to diffuse through the metal and then back again when compared to anon embedded drive, scanning method. In another embodiment with oneembedded sensor or sensitive material, protective layers are used tolimit fretting damage at the faxing surface.

A variety of sensors can be used to measure the response of the statesensitive materials. For example, for insulating or weakly conductingmaterials such as fiberglass composites, capacitive or dielectricsensors can be used. The sensors are the electromagnetic dual to theinductive sensors, with electric fields taking the place of magneticfields for inspecting the materials. A representative single sidedsensor geometry is shown in FIG. 35. The application of a sinusoidallyvarying potential of complex magnitude v and angular frequency ω=2πfresults in the flow of a terminal current with complex amplitude I,whose magnitude and phase is dependent on the complex permittivity ofthe material. The capacitive sensor 100 in one preferred embodiment hasinterdigitated electrodes as presented in U.S. Pat. Nos. 4,814,690 and6,380,747 and in U.S. patent application Ser. Nos. 10/040,797, filedJan. 7, 2002, and 10/225,406, filed Aug. 20, 2002, the entire teachingsof which are hereby incorporated by reference. This sensor 102 utilizesa pair of interdigitated electrodes 104 and 106 to produce a spatiallyperiodic electric field. The electrodes are adjacent to the material ofinterest with an insulating substrate and a ground plane on the otherside of the substrate. One of the two electrodes, 104, is driven with asinusoidally varying voltage, v_(D), while the other, 106, is connectedto a high-impedance buffer used to measure the magnitude and phase ofthe floating potential, v_(S). The periodicity of the electrodestructure is denoted by the spatial wavelength λ=2 π/k, where k is thewavenumber.

In another embodiment sensor diagnostics and/or recalibration isperformed for individual surface mounted or embedded sensors or networksof sensors by performing an act or exposing the sensor to a conditionchange that changes one or more unknowns while not changing at least oneother unknown (e.g., lift-off). In one such method the temperature isvaried using a heat source or measurements are made at different ambienttemperatures. This changes conductivity and or permeability withlift-off constant. In another such method, stress (or external load) isaltered to again vary conductivity or permeability of the MUT or addedsensitive material with lift-off and layer thicknesses constant. Inanother embodiment another layer is added keeping all else constant. Inanother embodiment the scanning of a drive or sense array relative to anembedded drive or sense array is accomplished at different lift-offs,temperatures or applied stresses. In still another method a bias fieldis used to change the permeability, including directional variations ifpractical. Both for performance check and measurement enhancements. Thesensor performance is verified by comparing to expected behavior.Recalibration is performed relative to expected results by monitoringtemperature or stress with thermostats/thermocouples or strain gages orsome other means or model prediction relative to load/source of change.

While the embodiments of the invention have been particularly shown anddescribed with reference to preferred embodiments thereof, it will beunderstood to those skilled in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the embodiment of the invention as defined by the appended claims.

REFERENCES INCORPORATED BY REFERENCE IN THEIR ENTIRETY

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Hydrogen in Metals, Proceedings of the Second Japan Institute of Metals,International Symposium, 1979.

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1. A method for inspection of magnetizable materials, said methodcomprising: disposing a sensor having at least two parallel driveconductors with at least one sense element on a substrate proximate to asurface of a test material at more than one location; passing atime-varying electric current through the drive conductor to create amagnetic field; measuring the response from each sense element, fordifferent sensor orientations, to determine magnetic permeability; and,correlating this magnetic permeability with an overload effect.
 2. Themethod as claimed in claim 1 wherein the response is the permeabilitydistribution over the material surface.
 3. (canceled)
 4. The method asclaimed in claim 1 wherein the response is measured at selectedlocations on the material.
 5. The method as claimed in claim 1 whereinthe sensor has a plurality of sense elements.
 6. The method as claimedin claim 5 wherein the sense elements are aligned in a directionperpendicular to the direction of sensitivity for the sensor.
 7. Themethod as claimed in claim 5 wherein the sensor is scanned over thesurface of the material.
 8. The method as claimed in claim 7 wherein thesensor is scanned with different orientations.
 9. The method as claimedin claim 5 wherein the sensor is mounted to the surface of the material.10. The method as claimed in claim 9 further comprising at least oneadditional sensor mounted in a different orientation than the firstsensor.
 11. The method as claimed in claim 1 wherein the test materialis a landing gear of an aircraft.